This invention relates to an elastic fluid axial flow turbine or compressor and, more particularly, to the shrouding used for the blading for such a turbine or compressor.
An elastic fluid axial flow turbine or compressor comprises a rotor having a peripheral groove and an annular row of blades having root portions disposed in the groove. A variety of blade structures are known and a variety of techniques for minimizing vibratory stresses in the blade structures have been developed. For example, U.S. Pat. No. 3,795,462 to Trumpler, Jr. describes the lashing of turbine blades into groups of three or more blades with a small V-shaped gap between the grooves. This gap tends to close due to untwisting of the blades when they are subjected to high centrifugal forces, thereby providing a substantially continuous lashing ring with vibration dampening characteristics.
A problem with short arc groups as shown by Trumpler is that low orders of harmonic excitation are insufficiently attenuated to prevent fatigue failure, particularly when wet and corrosive steam is used. To minimize this problem, the rotor blades have been connected together in long arc groups of more than 12 blades per group as described in my prior U.S. Pat. Nos. 3,588,278 and 4,386,887.
Although long arc shrouding has been successful in eliminating failure of untuned blading, failure of tuned blading can occur because of gaps between the groups of blades. It is necessary to leave gaps because of thermal and centrifugal stresses that develop in use. Because of the gaps, axio-torsional modes of vibration can develop high vibration amplitudes which can lead to failure of blading.
My prior U.S. Pat. No. 4,386,887 describes a solution to this problem, where the adjacent end blades of the long arc groups are flexibly tied together such as with S-shaped wire clips, pins, or sleeves. However, a problem with these connectors is that they introduce another element into the blade structure, thereby providing another element that can fail in use. Further, the pin or other tie means can work loose from the high stresses incurred by turbine blade structures.
Thus, there is a need for a technique for flexibly interconnecting the shroud segments of long arc groups together where the technique does not require additional parts that are subject to failure. Further, it would be desirable that the technique be useful for retrofitting existing turbines and compressors.